The invention re for reproducing data that can be read by an optical pick-up from the tracks on a recorded base in that at least one beam of light can be focused on the base by a control loop and positioned along the tracks by a tracking loop and in that the base reflects the beam onto a photodetector, which generates an electric data signal, a focusing-error signal that indicates the actual state of the focusing loop, and a tracking-error signal that indicates the actual state of the tracking loop.
Devices of this type, which are employed in compact-disk players, optico-magnetic equipment for recording and playback, equipment for recording and playing back draw disks, and videodisk players for example, are provided with an optical pick-up consisting of a laser diode, several lenses, a prism-based beam splitter, and a photodetector. The design and function of such an optical pick-up are described on pages 209-15 of Electronic Components and Applications, Vol. 6, No. 4 (1984).
The beam of light emitted by the laser diode is focused on the compact disk by lenses and reflected onto the photodetector from the disk. The data recorded on the disk is obtained along with the focusing-loop state and the tracking-loop state from the photodetector output signal. The aforesaid literature calls the focusing-loop state signal the "focusing error" and the tracking-loop state signal the "radial tracking error."
The component that controls the focusing loop is a coil with an objective lens that travels along its optical axis over the magnetic field. The focusing loop maintains the beam of light emitted by the laser diode constantly focused on the compact disk by linearly displacing the lens. The tracking loop, which is also often called a "radial drive mechanism," linearly displaces the optical pick-up along the radius of the compact disk, positioning the beam of light along the spiraling tracks on the disk.
The radial drive mechanism in some equipment comprises what are called a coarse-adjustment mechanism and a fine-adjustment mechanism. The coarse adjustment mechanism can for example be a spindle that radially displaces the whole optical pick-up, consisting of the laser diode, the lenses, the beam splitter, and the photodetector. The fine-adjustment mechanism can tilt the beam of light radially, at a prescribed angle for example, in order to advance just the beam by itself slightly along one radius of the compact disk.
Unobjectionable reproduction of the data, whether video and audio in the case of a videodisk player or audio alone in the case of a compact-disk player, demands, in addition to precisely focusing the beam of light on the videodisk or compact disk, precise positioning along the tracks on the disk.
The variable-gain amplifier in the focusing loop, however, is, like any variable-gain amplifier, subject to offset voltage to an extent that depends not only on temperature but also on chronic drift. The offset-voltage drift derives, along with the drift associated with other amplifier parameters, from the aging of the component.
To prevent data reproduction from being detrimentally affected by the offset voltage associated with the variable-gain amplifier, the voltage must be compensated. Manual adjustment of an equilibrating potentiometer in the control loop, however, can only be employed for approximate compensation because changes in the offset voltage due to temperature fluctuations and aging on the part of the amplifier are not allowed for.